In addition to artifact suppression and removing solvent peaks, gradient pulses can be used for coherence selection. Traditionally, this was done by phase cycling but using gradients for coherence selection allows cleaner spectra to be obtained more quickly. In this post the use of gradients to select coherences in heteronuclear experiments is discussed.
Thursday, December 7, 2017
Tuesday, November 14, 2017
The previous post described how a combination of gradients and pulses can be used to help reduce artifacts in NMR spectra. Adding a selective pulse to that sequence enables solvent signals to be excluded. This technique, sometimes known as excitation sculpting, is described below.
Tuesday, September 5, 2017
The previous post gave an introduction to gradient pulses, describing what they are. In this post, and the next few, I will describe a few applications of gradients. This post will describe how they can be used to reduce artifacts from imperfect pulses, allowing phase cycles to be reduced and experiments to be run more quickly.
Monday, August 7, 2017
Gradients have become an essential part of modern NMR spectroscopy. Nearly all probes now have pulsed field gradient capabilities and the vast majority of pulse sequences use gradient pulses. In this post, and those following, I will try and explain what gradients are and how they can be useful.
Sunday, July 2, 2017
Recently I have noticed many users recording spectra with low values for the receiver gain. The receiver gain is a scaling factor for the FID signal that ensures spectra are not distorted if the signal is too large, or resolution is lost if the signal is too small. Typically the receiver gain is set automatically by the command "rga", but users should pay attention to the value obtained as it is a good indicator of how successful the experiment will be. To demonstrate this I measured sensitivity at different receiver gain values.
Friday, June 2, 2017
The HMBC experiment is normally used to detect 1H-13C correlations, but it can also be used for 15N. An 15N HMBC experiment may provide structural information that cannot be obtained from a 13C HMBC, and it can also be used to determine the number of nitrogen atoms present in a compound. Recently I was asked, "How does the sensitivity of an 15N HMBC compare with that of a 13C HMBC?", so I decided to try and find out.
Monday, May 8, 2017
1D 13C spectra are often used to confirm compound identity. The simplicity of such spectra enable the number and type of carbon atoms present to be quickly evaluated. 2D 1H-13C correlation spectra, while inherently more sensitive and informative, require more work to interpret and may not show signals from all carbon atoms. Consequently, 1D 13C spectra still have a place in compound characterization. To increase the sensitivity and information content of 1D 13C spectra several variants of the original pulse sequence have been developed. Several of these are discussed below.